The present invention relates to a pneumatic radial tire in which a carcass ply (carcass layer) is constructed by a semiradial structure, and, more particularly, to a pneumatic radial tire promoting driving stability without being accompanied by an increase in mass or a reduction in productivity of the tire.
In pneumatic radial tires, a tire in which a carcass cord of a carcass ply is arranged at an angle less than 90 degrees relative to a tire equator is particularly referred to as a semi-radial tire. Generally, according to a conventional semi-radial tire for a passenger vehicle, as shown by FIG. 3A, two carcass plies 21 and 22 in which the carcass cords intersect with each other between the plies, are arranged to reach the bead portions 25 of the tire from a tread portion 23 via left and right side portions 24. Both end portions are folded back to wrap the bead fillers 27 from the inner side to the outer side of the tire respectively around left and right bead cores 26. Further, the tread portion 23 is arranged with two belt layers 28 over the entire periphery of the tire along the peripheral direction of the tire on the outer side of the outer side carcass ply 21.
In the carcass plies 21 and 22, a fold back portion 21b of the carcass ply 21 constituting the outermost side on an inner side of the bead core 26 in the bead portion 25, is extended to the side of the tread portion 23 along an unfolded portion 21a of the carcass ply 21 (that is, main body portion). Further, a fold back portion 22b of the carcass ply 22 rides over a distal end 21e of the fold back portion 21b to cover the entire fold back portion 21b and is successively extended to the side of the tread portion 23 along the main body portion 21a. Therefore, a distal end 22e of the fold back portion 22b is disposed further radially toward the tread portion 23 relative to the distal end 21e of the fold back portion 21b.
According to the above-described conventional semi-radial tire, when it is intended to improve the driving stability, as a general conventional method, a reinforcement layer comprising steel cords or fiber cords is arranged on the outer side of the bead filler 27. However, such an additional arrangement of the reinforcement layer gives rise to an increase in a mass of the tire by that amount. This increases also the number of tire fabrication steps and thus results in a disadvantage resulting in a reduction in the productivity of the tire.